From clarke@sunshine.sprl.umich.edu Tue Jul 5 14:44:28 1994 From clarke@sunshine.sprl.umich.edu Tue Jul 5 14:44:28 1994 John, We have beautiful UV Images of Io, at good exposure levels down to F218W plus weakly exposed F160W images due to faintness of emissions. We see Io at all wavelengths down to F160W. Following will be Gilda's DPS abstract, which describes the combined GTO/GO program we are pursuing: Ultraviolet Observations of Io with HST: WFPC2 Imaging and GHRS and FOS Spectroscopy G.E. Ballester, J. Clarke (U. of Michigan), J. Trauger, K. Stapelfeldt, D. Crisp (JPL), the WFPC2 Investigation Definition Team, J. Ajello (JPL), M. Combi (U. of Michigan), M. McGrath (STScI), N. Schneider (U. of Colorado) and D. Strobel (Johns Hopkins U.) A number of ultraviolet observations of Io with the Hubble Space Telescope were made in June 1994 to study the main constituents of Io's atmosphere, molecular SO2 and neutral oxygen and sulfur, and the surface reflectivity. Images were obtained with the Wide Field Planetary Camera 2 (WFPC2) as part of the WFPC2 Guaranteed Time Observer (GTO) program. A Guest Observer (GO) program was dedicated to disk-integrated spectroscopic observations with the Faint Object Spectrograph (FOS), and spectroscopic mapping with the Goddard High Resolution Spectrograph (GHRS). Images with a series of filters centered at ~ 2180, 2550, 3360 and 3800 A show the distribution of the surface reflectance and also of atmospheric SO2 gas absorption. As part of the GO program, disk-integrated spectra were obtained with the FOS that measure the SO2 atmospheric abundance from the ~2100 A absorption signature. Comparison of these two sets of data should add to their value in determining the spatial distribution of the main component of Io's atmosphere. The GO program also studied Io's neutral O and S components with spatial scans of their far-UV emissions with the GHRS. These scans should provide valuable information on the spatial distribution of the emissions. East/west and north/south scans of both optically thick and optically thin emissions were made, and depending on the details of the observing sequences, complimentary information may be obtained on the atmosphere and the dependence of the emissions on the Io plasma torus. WFPC2 images of Io were also obtained using the sodium Wood's filter covering ~1150-2100 A (with very low red-leak) for detection of neutral O and S far-UV emission features. The far-UV sunlight reflected by the surface is also measured in these images. Preliminary results will be presented. Re: future imaging, we have more images with WFPC 2 in our GTO program at both elongations in cycle 5, using filters F380 down to F160W, so I am afraid these are protected for one more cycle. The visible of course is wide open. We should talk about the comparison of our images toward developing a complete picture of Io, however, as we work on the respective data sets. p.s. the images we have from June 1994 are near west elongation. Regards, John --------------------------------------------------------------------------- IJW SATELLITE WORKSHOP AT DPS I'm planning a lunchtime meeting of the IJW Satellites Discipline during the DPS meeting in Annapolis in November. The meeting will be on a weekday, but the date is not set yet. Details later! --------------------------------------------------------------------------- OBSERVING IO'S VOLCANOS DURING THE COMET CRASH PERIOD For those with infrared cameras or photometers who have time during the impact observations, there is a chance to do some useful monitoring of Io's volcanos. Io will be eclipsed and occulted by Jupiter at the times in the following table. Time-resolved infrared photometry (2.3 microns or longer) of Jupiter occultation egress, which occurs in Jupiter's shadow, will give a 100-km resolution view of the spatial distribution of volcanic thermal emission across Io. Time resolution of a second or slower is useful. With the methane filters that everyone has handy for the comet crash, this is an easy observation at 2.3, 3.5, or 3.8 microns, and is good practice for the similar time-critical crash observations. Io emerges from behind Jupiter about 14 arcsec E, 11 arcsec S, from the center of the disk. It's also worth observing the Jupiter occultation ingress, which occurs in sunlight, especially at the longer wavelengths, as the occultation of bright volcanic hot spots, if any, can be seen even in sunlight. Absolute calibration of the photometry is useful to have. During the period of > 1 hour that Io is visible in Jupiter's shadow after occultation egress but before eclipse egress, multicolor photometry of the volcanic thermal emission is also valuable, at wavelengths of 1.7 microns or longer. If seeing is very much better than 1 arcsec, it is possible to directly resolve individual hot spots on the 1 arcsecond disk during this period. In May there was a 5-10 fold brightening of the volcanic thermal emission that lasted less than a few weeks: here is a chance to look for similar dramatic changes, or more subtle ones, during the most intensive monitoring of the Jupiter system that is ever likely to occur. Send questions or results to John Spencer, spencer@lowell.edu ---------------------------------------------------------- Occultation Ingress Occultation Egress UT ------------------- ------------------ Eclipse Date Start End Start End Egress ---------------------------------------------------------- 7/15 02:11:04 02:14:52 04:22:20 04:26:09 05:40 7/16 20:39:20 20:43:08 22:50:39 22:54:27 00:09* 7/18 15:07:47 15:11:35 17:19:09 17:22:57 18:38 7/20 09:36:13 09:40:01 11:47:38 11:51:25 13:07 7/22 04:04:47 04:08:35 06:16:14 06:20:02 07:36 7/23 22:33:19 22:37:07 00:44:49* 00:48:36* 02:04* ---------------------------------------------------------- *Following Day Occultation times are accurate to about 1 second. Eclipse egress times are taken from the Astronomical Almanac and are good to about 2 minutes. --------------------------------------------------------------------------- LATEST IMPACT PREDICTIONS This is from Paul Chodas: